1. Field of the Invention
The invention relates to an anti-friction bearing, and more particular to an anti-friction bearing which is high in resistance to impact, improved in load capacity, and has less tendency to produce increased sound levels as it ages; and a method for fabricating the same.
2. Description of the Related Art
Anti-friction bearings have been conventionally used for rotating parts in various mechanisms. In recent years, due to the rapid diffusion of office automation equipment, the ball bearing type of anti-friction bearing has increased in importance as the bearings for electric devices used in office automation equipment such as video cassette recorders (VCRs), floppy disk drives (FDDs), hard disk drives (HDDs), and scanner motors, to such an extent that the characteristics of the ball bearings influence the characteristics of the equipment. For example in the case of driving 3.5 inch HDDs., the bearings used for the motors cannot cope with this amount of downsizing unless they have a diameter of 15 mm or less. Moreover, even those to be used for VCRs and scanner motors require a diameter not greater than 30 mm.
As described above, ball bearings have become smaller in outer diameter to meet the shrinking size of office automation equipment. In addition, with the miniaturization and the increase in track density of HDDs, the ball bearings used therein must meet more severe requirements for improved rotational accuracy (with long-term retention of this rotational accuracy), improved impact resistance and load capacity. Especially, to meet the recent trend for Personal Computer (PC) notebooks and mobile computing, further improvements are required in impact resistance and load capacity.
For higher load capacities, the material constituting the ball bearing should be harder. For ball bearings constructed conventionally from Type 2 high carbon chromium bearing steel (SUJ2), tempering is normally performed in the temperature range 160 to 200xc2x0 C. in order to give the ball bearings a hardness of HRC58 or more. However, although such low tempering temperatures result in higher hardness, they also yield a great amount of retained austenite. This austenite is unstable, and there is a high possibility that the austenite will be transformed into martensite during use, producing strain which will increase noise levels.
To lower austenite levels, the tempering temperature has conventionally been raised, even though the ball bearing is therefore softer and has a reduced load capacity, in order to reduce the amount of retained austenite and minimize changes with age.
The present invention aims to improve such conventional disadvantages, and an object thereof is to provide a bearing to be used in office automation equipment and the like, wherein the impact resistance and load capacity are improved, the rotational accuracy will not deteriorate in long-term use and the quietness can be maintained over a long period of time.
Bearings are used under various conditions and at various temperatures, and they must not change their dimensions (measured at a given temperature) even after having been used at various temperatures. However, even a bearing made from steel hardened at an appropriate temperature and tempered properly can suffer great dimensional changes in a short period of time when, for example, the bearing is operated beyond its maximum operating temperature. These dimensional changes are also influenced by stresses imposed on portions of the bearing. A cause of the dimensional changes is the retained austenite in the bearing material.
That is, in hardened bearing steel, the retained austenite appears to decompose under the influence of temperature and time, expanding in volume to produce dimensional changes. For example, in a case where a bearing in an operating HDD is subjected to extreme impacts or loads for even a short period of time, frictional heat elevates the temperature of the affected portion beyond the upper limit of the design specification temperature, and the heat decomposes the retained austenite to rapidly produce dimensional changes. Moreover, even in cases where the elevated temperature does not exceed the specification, the application of excessive stresses causes the decomposition of retained austenite to produce dimensional changes over time. These dimensional changes cause production of noise from the bearing as well as shorten the life of the bearing.
In a conventional small size bearing having an outer diameter of 30 mm or less, Type 2 high carbon chromium bearing steel (SUJ2), is normally used as both the the bearing ring and the ball bearing. The chemical compositions of SUJ2 are as follows:
C: 0.95-1.10 percent by weight (Wt %),
Si: 0.15-0.35 wt %,
Mn: 0.50 wt % or less,
P: 0.025 wt % or less,
S: 0.025 wt % or less, and
Cr: 1.30-1.60 wt %.
When the above material is quenched and them tempered at a temperature between 160 and 200xc2x0 C., the amount of retained austenite in the bearing is as high as 8-14 by volume %. Thus, a bearing using Type 2 high carbon chromium steel suffers from great dimensional changes resulting from the aforesaid decomposition of retained austenite . When the bearing is used as a bearing for a HDD, stable use over a long period of time cannot be expected of the bearing. To avoid such problems, retained austenite may be reduced by raising the tempering temperature or by repeating the tempering process, as described in Japanese Patent Laid-Open Publication No. Hei 7-103241. However, like the repetition of the tempering over a plurality of times, the processing steps are complicated, which inevitably increases the man-hours required.
Accordingly, in the present invention, as compared with the chemical composition of the SUJ2, Mn (an austenite stabilizer) can be increased to form retained austenite which is stabler even at lower tempering temperatures, Si, which has high temper softening resistance, is added in increased amounts to suppress the lowering hardness at higher temperatures . Lastly, the range of the Cr content is enlarged in order to widen the quenchability range.
The chemical compositions of the high carbon chromium bearing steel to be used in the present invention are as follows:
C: 0.90-1.30 wt %,
Si: 0.40-1.20 wt %,
Mn: 0.90-1.60 wt %,
Cr: 0.90-1.70 wt %, and impurities of
P: 0.025 wt % or less, and
S: 0.025 wt % or less.
In view of the technical circumstances as described above, the invention according to claim 1 of the present application provides an improved anti-friction bearing. The improved bearing functions by means of rolling between portions supporting a load. The portions are in relative motion to each other. An inventive anti-friction bearing having an outer ring of 30 mm or less in diameter, wherein the high carbon chromium bearing steel constituting said anti-friction bearing is made from material which includes the following compositions, and which has an amount of retained austenite in its surface layer of less than 6% by volume.
C : 0.90-1.30 wt %,
Si: 0.40-1.20 wt %,
Mn: 0.90-1.60 wt %, and
Cr: 0.90-1.70 wt %.
In addition, another example of the present application provides a method for fabricating an anti-friction bearing, being a bearing functioning by means of rolling movements between portions supporting a load and which are in relative motion to each other, said anti-friction bearing having an outer ring of 30 mm or less in diameter, wherein the high carbon chromium bearing steel constituting the anti-friction bearing uses a material including
C: 0.90-1.30 wt %,
Si: 0.40-1.20 wt %,
Mn: 0.90-1.0 wt %, and
Cr: 0.90-1.70 wt %; and
where weight determinations are performed after quenching, at a tempering temperature in the range of 170xc2x0 C.-250xc2x0 C., the amount of retained austenite in its surface layer is less than 6 by volume % after the tempering, and the surface hardness is HRC60 or greater.